Video Player: Set proper aria-label to the toggle button for subtitles.

ui/file_manager/video_player/js/media_controls.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview MediaControls class implements media playback controls
  * that exist outside of the audio/video HTML element.
  */
 
 /**
@@ skipping 518 matching lines @@
   }
 };
 
 /**
  * Initializes subtitles button.
  */
 MediaControls.prototype.initSubtitlesButton = function() {
   this.subtitlesTrack_ = null;
   this.subtitlesButton_ =
       this.createButton('subtitles', this.onSubtitlesButtonClicked_.bind(this));
-  this.subtitlesButton_.setAttribute('aria-label',
-      str('VIDEO_PLAYER_SUBTITLES_BUTTON_LABEL'));
 };
 
 /**
  * @param {Event} event Mouse click event.
  * @private
  */
 MediaControls.prototype.onSubtitlesButtonClicked_ = function(event) {
   if (!this.subtitlesTrack_) {
     return;
   }
   this.toggleSubtitlesMode_(this.subtitlesTrack_.mode === 'hidden');
 };
 
 /**
  * @param {boolean} on Whether enabled or not.
  * @private
  */
 MediaControls.prototype.toggleSubtitlesMode_ = function(on) {
   if (!this.subtitlesTrack_) {
     return;
   }
   if (on) {
     this.subtitlesTrack_.mode = 'showing';
     this.subtitlesButton_.setAttribute('showing', '');
+    this.subtitlesButton_.setAttribute('aria-label',
+        str('VIDEO_PLAYER_DISABLE_SUBTITLES_BUTTON_LABEL'));
   } else {
     this.subtitlesTrack_.mode = 'hidden';
     this.subtitlesButton_.removeAttribute('showing');
+    this.subtitlesButton_.setAttribute('aria-label',
+        str('VIDEO_PLAYER_ENABLE_SUBTITLES_BUTTON_LABEL'));
   }
 };
 
 /**
  * @param {TextTrack} track Subtitles track
  * @private
  */
 MediaControls.prototype.attachTextTrack_ = function(track) {
   this.subtitlesTrack_ = track;
   if (this.subtitlesTrack_) {
@@ skipping 463 matching lines @@
   } else {
     this.container_.removeAttribute('fullscreen');
   }
 
   if (this.fullscreenButton_) {
     this.fullscreenButton_.setAttribute('aria-label',
         fullscreen ? str('VIDEO_PLAYER_EXIT_FULL_SCREEN_BUTTON_LABEL')
                    : str('VIDEO_PLAYER_FULL_SCREEN_BUTTON_LABEL'));;
   }
 };